Cellular adaptive immunity is mediated by T-lymphocytes, also known as T-cells, which upon recognition of a non-self or tumoral antigen can either destroy the target cell or orchestrate an immune response with other cells of the immune system.
Adoptive immunotherapy, which involves the transfer of autologous antigen-specific T cells generated ex vivo, is a promising strategy to treat viral infections and cancer. The T-cells used for adoptive immunotherapy can be generated either by expansion of antigen-specific T cells or redirection of T-cells through genetic engineering (Park, Rosenberg et al. 2011). Transfer of viral antigen specific T-cells is a well-established procedure used for the treatment of transplant associated viral infections and rare viral-related malignancies. Similarly, isolation and transfer of tumor specific T-cells have been shown to be successful in treating melanoma.
Novel specificities in T-cells have been successfully generated through the genetic transfer of transgenic T cell receptors or chimeric antigen receptors (CARs) (Jena, Dotti et al. 2010). CARs are synthetic receptors consisting of a targeting moiety that is associated with one or more signaling domains in a single fusion molecule. In general, the binding moiety of a CAR consists of an antigen-binding domain of a single-chain antibody (scFv), comprising the light and variable fragments of a monoclonal antibody joined by a flexible linker. Binding moieties based on receptor or ligand domains have also been used successfully. The signaling domains for first generation CARs are derived from the cytoplasmic region of the CD3zeta or the Fc receptor gamma chains. First generation CARs have been shown to successfully redirect T cell cytotoxicity, however, they failed to provide prolonged expansion and anti-tumor activity in vivo. Signaling domains from co-stimulatory molecules including CD28, OX-40 (CD134), and 4-1BB (CD137) have been added alone (second generation) or in combination (third generation) to enhance survival and increase proliferation of CAR modified T-cells. CARs have successfully allowed T-cells to be redirected against antigens expressed at the surface of tumor cells from various malignancies including lymphomas and solid tumors (Jena, Dotti et al. 2010).
While it is thus possible to redirect T-cell cytotoxicity towards tumor cells, these later cells may still dampen the immune response by escape mechanisms. One such escape mechanism is the elimination of certain amino acids such as arginine and tryptophan from their local microenvironment by production of arginase and Indoleamine 2,3-dioxygenase (IDO1).
Most reports have associated arginase activity with the need for malignant cells to produce polyamines to sustain their rapid proliferation. However, arginase tends to inhibit T-cell proliferation and activation.
Rodriguez et al. (2004) found that L-arginine (L-Arg) plays a central role in several biologic systems including the regulation of T-cell function. L-Arg depletion by myeloid-derived suppressor cells producing arginase I is seen in patients with cancer inducing T-cell anergy. They showed that L-Arg starvation could regulate T-cell-cycle progression insofar as T cells cultured in the absence of L-Arg are arrested in the G0-G1phase of the cell cycle. This was associated with an inability of T cells to up-regulate cyclin D3 and cyclin-dependent kinase 4 (cdk4). Silencing of cyclin D3 reproduced the cell cycle arrest caused by L-Arg starvation. They also found that Signaling through GCN2 kinase was triggered during amino acid starvation.
A recent study demonstrated that arginase is expressed and released from Leukemia blasts and is present at high concentrations in the plasma of patients with acute myeloid leukemia (AML), resulting in suppression of T-cell proliferation (Mussai, F. et al. 2013). The study showed that the immunosuppressive activity of AML blasts can be modulated through small molecule inhibitors of arginase and inducible nitric oxide synthase, strongly supporting the hypothesis that AML creates an immunosuppressive microenvironment that contributes to the pancytopenia observed at diagnosis. High arginase activity has been also described in patients with solid tumors, in particular in gastric, colon, breast, and lung cancers, and more particularly in small cell lung carcinoma (Suer et al., 1999). It is also considered that the following reaction catalyzed by arginase+H2O→urea+ornithine increases urea and ornithine concentration is the environment of tumors, which may have a negative impact on lymphocytes. On another hand, the inhibition of arginase in vivo was found to decrease tumor growth in mice as per the study by Rodriguez et al. (2004).
The metabolic enzyme IDO1 contributes to the balance between tolerance versus inflammation in a number of experimental models. Expression of IDO1 in APCs, such as macrophages and dendritic cells, can suppress T cell responses as observed during mammalian pregnancy, inflammatory conditions, autoimmunity and tumor resistance. IDO1 was found to be over-expressed by plasmacytoid dendritic cells in tumor draining lymph nodes (Munn, D. H. et al., 2004) as well as in child acute myeloid leukemia (AML) (Rutella, S. et al., 2013) and patients with chronic lymphocytic leukemia (Lindström V., et al. 2013). IDO1 catabolizes the essential amino acid tryptophan, thus decreasing concentrations in the local microenvironment as well as generating biologically active downstream metabolites. Studies in both yeast and mice revealed that GCN2 also plays a role in the response to tryptophan deprivation. PRDM1 (also referred to as BLIMP-1) is a protein, which expression level parallels that of IDO1, and that is up-regulated in situation of tryptophan deprivation.
It thus appears that production of arginase and/or IDO1, through amino acid deprivation, represents a significant component of tumor escape, which needs to be addressed by innovative immunotherapy strategies, especially those involving T-cells.